The present invention relates to tubular belt conveyors and, in particular, to a prestressed, self-closing tubular conveyor belt and to the components and to the overall construction of the production line which is used for making such prestressed tubular belt.
In my previous inventions, U.S. Pat. Nos. 4,823,941 and 5,836,440, it was suggested that the lateral prestressing of the multi-layer tubular belt of two or more layers can be secured when the layers are prestressed laterally and then joined together by gluing, welding, or by reinforcing the belt with a prestressed flexible spring-like carcass, or by a combination of these two approaches. Longitudinal edges are fabricated, respectively, as a tongue on one side and as a mating grove on another side. In one aspect, this belt comprises elastic inner and outer layers which are permanently joined together so that the inner layer is in tension across its width before and after fabrication, and the outer layer was not prestressed before fabrication but is in compression across its width after fabrication.
In this configuration, the inner prestressed forces wind the flat belt into a tubular configuration after the multi-layer belt is cured. The inner bending moment is large enough to bend the belt laterally through 360° so that the edges are always under compression, no matter what the configuration of the conveyor path is, what the speed of movement is, or what the loading condition of the conveyor belt is.
This known method of fabrication, as described above, requires preparing two flat layers of elastic material of different width. The inner layer (smaller width) is prestressed in the lateral direction by tensile forces distributed along longitudinal edges of the layer; it is stretched to the larger width of the outer layer. Next, the two layers are tied together by applying pressure perpendicular to the plane of the layers, and simultaneously gluing or welding them to form the composite two-layered tubular belt. Then the tensile forces along longitudinal edges of the inner layer and the pressure perpendicular to the layers' plane are removed, allowing the released belt to shrink to its normal tubular configuration.
The technology of fabricating the prestressed tubular belt addressed the fundamental principle of forming a tubular shape using the above-described stressing mechanism. However, when creating the shape of the tube, a problem results when trying to balance the shape of the tube with providing a compression force across the belt longitudinal joint. For example, for having a circular cross section after prestressing and for a compression force across the belt longitudinal joint, one might have either a circular shape with nearly no compression forces along the belt joint, or, by increasing the constant lateral stress for achieving the predetermined compression in the joint, one might achieve a distorted shape of the cross section, usually resulting in the edges curling inside toward the center of the tube.